1. Field of the Invention
The present invention relates to a catalyst, for purifying an exhaust gas, used for the purification of the exhaust gas emitted from internal combustion engines of automobiles and the like and, more specifically, it relates to a catalyst for purifying an exhaust gas with a notably improved NOx purifying performance at high temperature.
2. Description of the Related Art
Reduction of total carbon dioxide (CO2) emissions and nitrous oxide (NOx) production from internal combustion engines of automobiles and the like have become worldwide issues from the standpoint of protecting the earth""s environment. Some measures being taken include the development of xe2x80x9clean burnxe2x80x9d engines for improved mileage and the development of NOx purifying catalysts of an absorption-reduction type that provide conventional three-way catalysts with the additional function of absorbing NOx in a lean atmosphere for purification of the exhaust gas, and a certain degree of success has thereby been achieved against the aforementioned problems.
Lean burn engines ordinarily burn fuel under conditions with a xe2x80x9cleanxe2x80x9d air/fuel (A/F) ratio (air excess), and for limited periods burn fuel under stoichiometric (theoretical air/fuel ratio) to rich (fuel excess) conditions.
The hydrocarbons (HC) and carbon monoxide (CO) in an exhaust gas are efficiently removed by combustion due to the effect of the oxidizing atmosphere and catalyst at the lean end, while NOx is occuluded in the absorbent at the lean end and temporarily released under stoichiometric to rich conditions, and is purified by reduction due to the effect of the temporary reducing atmosphere and the catalyst.
Alkali metals or alkaline earth metals have conventionally been used as the NOx absorbents of NOx purifying catalysts of an absorption-reduction type, and the catalysts for purifying an exhaust gas are constructed by loading a catalyst component such as platinum and a carbonic acid salt or acetic acid salt of an alkali metal or alkaline earth metal on a carrier such as xcex3-alumina. Such catalysts for purifying an exhaust gas are described, for example, in Japanese Unexamined Patent Publications No. 9-248458, No. 10-33984 and No. 10-128114.
On the other hand, specific complex oxides of the perovskite type are known to have the effect of decomposing NOx into N2 and O2, and NOx catalytic reduction type catalysts comprising specific complex oxides of the perovskite type carried on carriers are described, for example, in Japanese Unexamined Patent Publications No. 5-261289, No. 5-245372 and No. 6-315634.
Nevertheless, NOx purifying catalysts of the absorption-reduction type carrying alkali metal salts or alkaline earth metal salts using carbonic acid salts or acetic acid salts still have room for an improved NOx purifying performance at catalytic temperatures that are above approximately 500xc2x0 C.
Also, catalysts comprising conventional complex oxides of perovskite type are used for purification of exhaust gas emitted from internal combustion engines of automobile engines and the like, and are thus still need a higher NOx purifying efficiency.
It is therefore an object of the present invention to provide an improved catalyst for purifying an exhaust gas that can exhibit excellent NOx purifying performance even at catalytic temperatures exceeding approximately 500xc2x0 C.
The aforementioned object is achieved by a catalyst for purifying an exhaust gas characterized in that at least one catalyst component from among platinum (Pt), palladium (Pd), rhodium (Rh), gold (Au), silver (Ag) and iridium (Ir) is carried on a complex oxide of the perovskite type comprising at least two different metal elements.
That is, this invention is a catalyst comprising a catalyst component such as platinum directly carried on a specific complex oxide of the perovskite type. While the reason for the high NOx purifying performance of such a catalyst is not fully understood, it is theorized to be as follows.
Complex oxides of the perovskite type comprising at least two different metal elements generally produce distortion in crystals due to the difference in atomic radii of the metal elements contained therein, and it is believed that the structural oxygen defects of this crystal distortion have an activating effect on NOx, and that the activating effect works synergistically with the effect of the catalyst component such as platinum. This synergistic effect is believed to become significant when a catalyst component such as platinum is directly carried on the complex oxide of perovskite type.